1. Field of the Invention
This invention relates to the generation of video signals, and is more particularly concerned with the generation of a video signal which represents a composite image and is formed by combining plural digital input video signals.
2. Description of the Prior Art
In the art of creating digital video effects, a requirement may arise to create an effect in which two or more images or pictures appear to fly through one another in three-dimensional space. For example, referring to FIG. 1 of the accompanying drawings, a requirement may exist to move three individual pictures P1, P2 and P3 about a display screen S, for example a display screen of a cathode ray tube, so that they appear to fly though one another. FIG. 1 shows the resultant composite image or picture as displayed on the screen S as frozen for a particular instant in time when all three of the pictures P1, P2 and P3 intersect one another, and FIGS. 2, 3 and 4 show, on a reduced scale, the parts of the pictures P1, P2 and P3 that are displayed at that particular instant.
A known technique for creating such an effect will now be described with reference to FIG. 5 of the accompanying drawings. The technique employs three separate digital video effect (DVE) generators--also known as digital multi effect (DME) generators--DME1, DME2 and DME3. Three conventional digital video signals S1, S2 and S3, each comprising a series of samples representing successive picture cells (pixels), are supplied to the DME generators DME1, DME2 and DME3, respectively. The signals S1, S2 and S3 represent the pictures (planes) P1, P2 and P3, respectively, prior to manipulation, that is to say while they are still conventional pictures, namely undistorted and occupying the whole of a screen. The DME generators DME1, DME2 and DME3 process the signals S1, S2 and S3, respectively, so as to manipulate ( crop, reduce in size, rotate, shear, etc.) the pictures (planes) that they represent, on a field by field basis, to produce processed signals on video busses VB1, VB2 and VB3 that represent the manipulated pictures P1, P2 and P3. These processed signals, if viewed, would each comprise a respective one of the pictures P1, P2 and P3, each as shown in FIG. 1 except that it would be seen in full ( rather than having parts hidden by the other pictures), each picture being keyed (inset) into a neutral (for example, plain blue or black) background picture B corresponding to the full size of the screen S.
The video busses VB1, VB2 and VB3 are connected to respective inputs of a combiner CB, which combines together the three processed signals from the DME generators DME1, DME2 and DMEB. The combiner CB cannot simply mix the three processed signals since this would result in the parts of each of the pictures P1, P2 and P3 that should be invisible (hidden) being mixed with, rather than concealed by, the pictures that should be shown as being in front of them. To achieve the desired concealment, the combiner CB controls the combination of the signals on the video busses VB1, VB2 and VB3 by using information supplied to the combiner from Z-busses ZB1, ZB2 and ZB3 which extend from the DME generators DME1, DME2 and DME3 to the combiner. The information on the Z-busses ZB1, ZB2 and ZB3 represents, for each pixel of the signals on the video busses VB1, VB2 and VB3, the displacement behind the plane of the screen (depth) of that pixel of the associated picture, namely the position of that pixel in a Cartesian coordinate system in which the X and Y axes lie on the screen and the Z axis is orthogonal to the screen. The combiner CB compares the depth information on the Z-busses ZB1, ZB2 and ZB3 on a pixel by pixel basis and selects for output that pixel having the smallest value of Z, that is the pixel that should be closest to the viewer. That is, based on the depth information on the Z-busses ZB1, ZB2 and ZB3, the combiner performs the required hidden surface removal.
The technique described above is subject to two disadvantages. The first is that it necessitates the use of three DME (DVE) generators. This imposes a large cost penalty because DME generators are expensive even for conventional definition television systems, and even more so in the case of high definition television (HDTV) systems.
The second disadvantage is degradation of the quality of the composite picture along the edges where the individual pictures meet. This is due to the fact that the switching between the pictures is controlled, as explained above, by the depth information, and the depth information is generated at the sampling (pixel) frequency whereby the above-mentioned edges become jagged. This will now be explained in more detail with reference to FIG. 6 of the accompanying drawings, which shows a case similar to that of FIG. 1, but somewhat simpler in that only two pictures (planes), PA and PB, are combined, the two planes intersecting along an edge or boundary p1-p2 whereby the abovementioned problem of jaggedness occurs at that edge. The problem arises due to the spatial resolution of the pictures PA and PB. In this regard, the individual pictures PA and PB are represented by digital video signals which comprise samples representing respective cells (pixels) of the picture. That is, each picture can be considered to comprise an orthogonal array or grid of pixels, each horizontal row thereof being centred on a horizontal scanning line and the horizontal rows being spaced apart by the distance between the scanning lines. (See, in this regard, FIG. 7 of the accompanying drawings, which shows part of a composite picture like that of FIG. 6, but with an edge or boundary B1, corresponding to the edge p1-p2, orientated differently, divided into pixels p with the scanning lines represented at L). Thus, the spatial resolution is determined by the pixel size, which is in turn determined by the number of lines per field or frame of the video system employed.
Obviously, in general, when the pictures PA and PB are combined to form a composite picture, the edge B1 (p1-p2) at which the two pictures PA and PB intersect will not coincide precisely with pixel boundaries. Instead, in general, the edge will intersect pixels. Therefore, when the pictures PA and PB are combined together, a decision has to be made on the picture content of each pixel intersected by an edge. Thus, if, for example, the decision is to the effect that each such pixel will comprise either wholly one of the pictures PA and PB or wholly the other of the pictures depending upon whether the majority of that pixel should be occupied by the one picture or the other picture, respectively, the result is that the desired boundary between the pictures is in practice provided by a step-wise approximation thereto at pixel resolution. This can be more clearly appreciated by referring further to FIG. 7, in which the desired boundary (corresponding to the edge p1-p2) is shown by the line B1 and the step-wise approximation thereto by a line B2. Thus, the actual edge is jagged and there is aliasing between the pictures PA and PB. The degree of jaggedness becomes particularly noticeable when the edges are close to the horizontal or close to the vertical.